The present invention relates to a process for preparing succinylsuccinic acid diesters.
Succinylsuccinic acid diesters are available intermediates for quinacridone pigment. It is known that succinylsuccinic acid diesters are prepared by subjecting .gamma.-haloacetoacetic acid esters to dimerization in a reaction medium in the presence of an inorganic or organic base, and various combination of a reaction medium and a base has been investigated, for instance, (1) dimerization in an alcohol solvent such as ethanol in the presence of an organic base such as sodium ethylate and dimethylamine or an inorganic base such as sodium acetate and ammonia, described in Bull. Soc. Chim. France, 29, 402 (1921), (2) dimerization in an organic solvent having no hydroxyl group such as toluene and dimethyl sulfoxide in the presence of an organic base such as triethylamine and sodium ethylate or an inorganic base such as sodium hydroxide, described in Japanese Patent Disclosure No. 13147/1974, (3) dimerization in water in the presence of sodium ethylate, described in Bull. Soc. Chim. France, 29, 402 (1921), and (4) dimerization in an aqueous buffer solution of an inorganic base, described in Japanese Patent Disclosure No. 47349/1974. However, these known processes have the disadvantages that the yield of succinylsuccinic acid diesters is low and impurities which lower the quality of quinacridone prepared from the diesters are by-produced. For instance, the yield of the above process (1) is at most 29% and the process is not practical. The above process (2) requires an additional recovery step since a large amount of the produced diester is dissolved in the employed organic solvent and the dissolved diester must be recovered from the solvent to raise the yield. In case of the above process (3), the yield is at most 58% and this process is also impractical. In accordance with the above process (4), the diester is prepared in rather high yield, but the trace amount of impurities lowers the quality, especially chroma, of quinacridone prepared from the diester. Purification such as recrystallization to remove the inpurities decreases the yield of the diester due to dissolution of the diester in the mother liquor.